1. Field of the Invention
This invention relates to automatic focus adjusting devices suited to be used in video cameras or other video apparatus.
2. Description of the Related Art
For the automatic focus detecting device for use in the video camera or the like, many types have been proposed. In general, using the video signal obtained from the image sensor, focus adjustment is carried out by moving the photographic lens so as to bring the high-frequency component extracted from the video signal to a maximum. This is known as the so-called hill climbing method. Such an automatic focus adjusting type obviates the necessity of the special optical members which would otherwise be used for focus adjustment in the apparatus of sending and receiving infrared light or supersonic waves to and from a target. In addition, it has a merit in that for any target, or object to be photographed, however far or near it may be, focusing can be controlled with high accuracy independently of its distance.
For application to the case of driving the lens merely to move in a direction to increase the level of the high-frequency component, because, at the time of start, the lens has to be preliminarily moved in either direction to see which direction, higher or lower, the level of the high-frequency component changes to, other-wise the focusing movement could not be discriminated between the directions to and from an in-focus point. It is also proposed that the focusing lens or the image sensor is made to always minutely wobble back and forth and the resulting change of the level of the high-frequency component is used to get information representing the near-focus or far-focus state.
An automatic focus adjusting device of this kind is described using FIG. 1.
In FIG. 1, the automatic focus adjusting device comprises a first lens group 1 for focus adjustment (hereinafter called the "F lens"), a second lens group 2 for varying the focal length (hereinafter called the "V lens"), a third lens group 3 for performing compensation to keep constant the position of an image plane against zooming (hereinafter called the "C lens"), an iris 4 and a fourth lens group 5 for forming an image of an object right on an image sensing plane (hereinafter called the "RR lens"). Light passing through these lens groups is focused on the image sensing plane of an image sensor 6, where it is photoelectrically converted into an electrical signal and is output as the video signal. The image sensor 6 is arranged to be wobbled very short distances axially in predetermined periods by a drive circuit 15 whose operation is controlled in conformance with timing signals from a timing generating circuit 14. By this, the image sensing plane periodically vibrates back and forth. Responsive to this vibration, the image changes its focus state, which serves as a modulating signal to modulate the image sensing signal. The video signal output from the image sensor 6 is amplified to a predetermined level in passing through a pre-amplifier 7 and therefrom supplied to a camera signal processing circuit 8, where it is converted into a standard television signal by gamma correction, blanking, addition of a synchronizing signal, and other processing, and also to a band pass-filter (BPF) 9. In the BPF 9, a component of the video signal which varies with variation of the focus state, i.e., the high-frequency component, is extracted. A gate circuit 10 then extracts that portion of the input signal which corresponds to a focus detecting area set in a part of the area of a picture plane. A peak detection circuit 11 then detects a peak value of the extracted signal in a field or frame period. This peak value is then processed in a synchronous detection circuit 12 according to the output signal of the timing generating circuit 14. The output signal of the synchronous detection circuit 12 is supplied to an amplifier 13, where the loop gain is set to a particular value, and is therefrom supplied to a focus motor drive circuit 16. As a focus motor 17 is energized by the drive circuit 16, the F lens 1 is moved to adjust the focus.
Next, using FIGS. 2(a) and 2(b), the principle of automatic focus detection is explained. FIGS. 2(a) and 2(b) show respectively the variations of the output A of the peak detection circuit 11 and the output B of the synchronous detection circuit 12 with movement of the lens from the in-focus point to either of far and near points. The output A has a mountain-like characteristic curve which takes a maximum at the in-focus point and decreasing values as focusing goes to the far or near side. Because the image sensor 6, however, wobbles back and forth on the optical axis, as has been described, this is responded by adding either one of signals AN and AF of opposite phases on the near and far sides of the in-focus point, or a signal AM whose amplitude is a minimum at the in-focus point. To take this out, therefore, the same frequency as that of the wobbling is used in the synchronous detection. Thus, a signal that is of opposite signs for the near and far focus states and has a value of zero in sharp focus can be obtained at the output B, as shown in FIG. 2(b). This signal is appropriately amplified by the amplifier 13 and supplied to the motor drive circuit 16. In such a manner, discrimination between the near focus and the far focus is made to determine the direction in which the lens is to move to bring the image into sharp focus. After this, the lens can be moved at a speed corresponding to the detection output.
In a case where the present system employs the zoom lens, the degree of sensitivity of the image plane to the movement of the F lens varies depending on the focal length and the aperture size, i.e., the depth of field. In more detail, under the condition that the focal length is long (as in the telephoto positions), or the iris is at or near the full open aperture with which the depth of field is shallow, the output A changes like a steep mountain as shown in FIG. 3(a). So, the synchronous detection output B gives a clear in-focus point as shown in FIG. 3(b). Hence, the accuracy of stopping control takes a high absolute value. On the contrary, when the focal length is short (as in the wide-angle positions) and the iris is stopped down to deepen the depth of field, the output A is flattened like a gently sloping hill as shown in FIG. 3(c). So, the synchronous detection output B has a small level difference over the entire focusing range as shown in FIG. 3(d), giving unclear information for the in-focus point. Hence, in this condition, the absolute stop accuracy is lowered.
Therefore, the system of the character described above has the following drawback. Suppose, after the automatic focus adjustment has been done in the wide-angle position, the photographer turns a zoom actuator or switch 20 to the other position where another control voltage Vz is applied to a zoom motor drive circuit 18 so that a zoom motor 19 moves the V and C lenses to the telephoto end, then it becomes necessary to correct the stopped position for the in-focus state of the F lens in the middle of the course of zooming. If this correction is slow, the image is caused to blur transiently. This occurs likewise even when the zooming operation is carried out by hand.
In other words, when zooming from the telephoto side to the wide-angle side, the lens driving is in a direction to increase the depth of field. Therefore, it hardly happens that the image blurs. When zooming from the wide-angle side to the telephoto side, on the other hand, the depth of field decreases. A deviation of the plane of sharp image from the image sensing plane which could not be recognized in the wide-angle positions comes to appear as zooming approaches the telephoto end.